"When you take the number of the head(s) of the B'nei Yisrael ... and they shall give each an atonement (or: a substitute) to Hashem for his soul... This they shall give... a half-shekel" (30:12-13). Rashi and other commentaries note that from here we derive the prohibition against directly counting Jews. (This halacha is recorded by Rambam Hilchos T'midin U'Musafin (4:4) (although utilizing a different passage as the source) and Magen Avraham (O.C. 156).) R. Eliyahu of Vilna, in Aderes Eliyahu, and others note that included in these passages are two separate commandments: one, to always use some substitute when counting Jews; two, to use specifically a half-shekel as the substitute at this juncture to be used for the construction of the adanim, or sockets, which would serve as the foundation of the mishkan.
Ramban, in his commentary to Parshat Bamidbar, suggests that counting the entire Jewish people at the same time, as was done in the Midbar, was only permissible if the people over twenty were the only ones counted. It was an error concerning this halacha, opines Ramban, that led to a plague when King David counted all the Jewish people including those under twenty, as recorded in Shmuel (II Chapter 24). (See "The Halachic Controversy Concerning the Israeli Census", by Rav J. David Bleich, RJJ Journal of Halacha and Contemporary Society (VIII), for a summary of the extensive halachic discussion amongst contemporary Torah scholars concerning the applicability of these halachot to the Israeli Census.) The Torah here states "l'chapeir 'al nafshoseichem" -- "to atone for your souls" (30:16) in order to avoid "negef bifkod otam" -- "a plague when counting them" (30:12) implying that even when counting only those above twenty, there is still a need for kappara (atonement).
What might be the cause of the plague when counting the entire Jewish people including those under twenty? Why the need for a kappara when counting those over twenty? Rav Shaul Yisraeli z"l, the founding head of Kollel Eretz Chemda, (Shana B'Shana 5722) proposes a fascinating approach. He introduces his analysis by quoting the following passage from Yoma (22b). After quoting the verse in Hoshei'a (2:1): "v'haya mispar b'nei Yisrael k'chol hayam asher lo yimad v'lo yisafeir" -- "and the number of the Children of Israel shall be as the sand of the sea which cannot be measured and cannot be counted" as an additional source for the prohibition against counting Jews (see Rav Yisraeli's article as to why an additional source is necessary), the Talmud quotes the statement of R. Yonatan. He raises a contradiction between the first half of the passage which implies that there is a "mispar" to the Jewish people and the second half which states "which cannot be counted." The resolution he provides is that whereas when "'osin r'tzono shel makom" -- "they follow the Will of G-d" -- there is no number, but when "ein 'osin r'tzono shel makom" -- "they do not follow the Will of G-d" -- there is a defined number to them. Rav Yisraeli explains that herein lies the key to understanding the prohibition against counting Jews.
Our survival has never been determined by our number. Indeed, based on purely mathematical considerations, the small numbers of the Jewish People, as compared to the nations of the world who throughout history have attempted to destroy us, would have led to little chance of survival. But, "lo meirub'chem mikol ha'amim chashak Hashem bachem ... ki atem ham'at mikol ha'amim" -- "not because of your great numbers did Hashem desire you, for you are the smallest of the nations" (Va'Etchanan 7:7). On the contrary, in our small numbers lies our strength. "Am zu yatzarti li t'hilati y'sapeiru" -- "I have created this nation; they will recite my praise to me" (Yeshaya 43:21). Through Hashem's Providence over his beloved nation throughout the ages, His Name is sanctified amongst the nations. Gideon, the Judge of Israel, is even told to lessen the number of soldiers so as to increase the miracle occurring through them (Shoftim 7:4). On Chanukkah, in the 'al hanissim prayer, we recite "rabim b'yad m'atim" -- "the many in the hands of the few". We have never operated or survived based on the principal of numeric strength. "Yisrael b'tach baHashem" is the ever-present source of our victory against our enemies. The "mispar" of the B'nei Yisrael in the words of Hoshei'a is "that they have no number." Their strength lies in their devotion to their G-d and their faithful carrying of His Word to all of humanity, not in their numerical superiority. This explains the unique prohibition requiring a machatzit hashekel to atone for the counting of large segments of the 'Am Yisrael and, according to Ramban, the absolute prohibition of counting all of them. Over-reliance on the significance of numbers, ultimately an expression of a lack of reliance on G-d, can lead to a removal of His protection, or a plague. (See Ramban (ibid.) who suggests that counting for no reason other than to determine the number of Jews is also absolutely prohibited even with a substitute. This can be explained with similar reasoning.)
The Talmud Yerushalmi (Megilla 13b) comments on the statement of Haman in the Megilla "va'aseret 'alafim kikar kessef eshkol ... l'havi el ginzei hamelech" -- "and 10,000 talents of silver I will measure out ... to bring to the king's treasury." Hashem chose the month of Adar, the scheduled time of Haman's decree, as the time for bringing the machatzit hashekel in order to pre-empt the effects of the decree of Haman. "Hikdim shikleihem lish'kalav" -- "he preceded the B'nei Yisrael's sh'kalim to Haman's". Perhaps we can extend Rav Yisraeli's approach to elucidate this somewhat cryptic Midrash. By donating the half-shekel, K'lal Yisrael demonstrated their confidence in the inapplicability of numeric formulae in determining the survival of the Jewish People. In the time of Haman, when the entire kingdom of Achashveirosh was ordered to annihilate the relatively tiny Jewish Nation in all their places of dispersion, the merit of the earlier bitachon in Hashem's providence over his nation contributed to their salvation.
When once again the "wolves" seek to devour the "sheep" of Israel, we once again must turn and return with extra dedication to the service of our Protector, Tzur Yisrael V'Goalo, with full realization that only with His help, "heima kar'u v'nafalu, va'anachnu kamnu vanis'odad!"